Method for transmitting control information and apparatus for the same

ABSTRACT

A method for transmitting data in a wireless local area network, the method comprising: receiving, by a receiving station, a physical layer protocol data unit (PPDU) from a transmitting station, the PPDU including a signal-A field, the signal-A field including a bandwidth in which the PPDU is received, decoding the PPDU, wherein, when the PPDU is used for single-user (SU) transmission, the PPDU is decoded with assuming that the PPDU does not include a signal-B field, and, when the PPDU is used for multi-user (MU) transmission, the PPDU is decoded with assuming that the PPDU include the signal-B field, and wherein the signal-B field includes a modulation and coding scheme (MCS) used for the receiving station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.15/060,727 filed on Mar. 4, 2016, which is a Continuation of U.S. patentapplication Ser. No. 14/227,993 filed on Mar. 27, 2014 (now U.S. Pat.No. 9,300,512 issued on Mar. 29, 2016), which is a Continuation of U.S.patent application Ser. No. 13/520,769 filed on Jul. 5, 2012 (now U.S.Pat. No. 8,718,173 issued on May 6, 2014), which is filed as theNational Phase of PCT/KR2011/000860 filed on Feb. 9, 2011, which claimsthe benefit under 35 U.S.C. § 119(e) to U.S. Provisional ApplicationNos. 61/375,299 filed on Aug. 20, 2010, 61/307,429 filed on Feb. 23,2010, and 61/303,684 filed on Feb. 12, 2010, all of which are herebyexpressly incorporated by reference into the present application.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method for transmitting control information with highreliability in a wireless local area network (WLAN) system and anapparatus supporting the method.

Discussion of the Related Art

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

Ever since the institute of electrical and electronics engineers (IEEE)802, i.e., a standardization organization for WLAN technologies, wasestablished in February 1980, many standardization works have beenconducted.

In the initial WLAN technology, a frequency of 2.4 GHz was usedaccording to the IEEE 802.11 to support a data rate of 1 to 2 Mbps byusing frequency hopping, spread spectrum, infrared communication, etc.Recently, the WLAN technology can support a data rate of up to 54 Mbpsby using orthogonal frequency division multiplex (OFDM). In addition,the IEEE 802.11 is developing or commercializing standards of varioustechnologies such as quality of service (QoS) improvement, access pointprotocol compatibility, security enhancement, radio resourcemeasurement, wireless access in vehicular environments, fast roaming,mesh networks, inter-working with external networks, wireless networkmanagement, etc.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing rate of up to above 540 Mbps, and is based on amultiple input and multiple output (MIMO) technique which uses multipleantennas in both a transmitter and a receiver to minimize a transmissionerror and to optimize a data rate. In addition, this standard may use acoding scheme which transmits several duplicate copies to increase datareliability and also may use the OFDM to support a higher data rate.

An IEEE 802.11n HT WLAN system employs an HT green field physical layerconvergence procedure (PLCP) protocol data unit (PPDU) format which is aPPDU format designed effectively for an HT station (STA) and which canbe used in a system consisting of only HT STAs supporting IEEE 802.11nin addition to a PPDU format supporting a legacy STA. In addition, anHT-mixed PPDU format which is a PPDU format defined such that a systemin which the legacy STA and the HT STA coexist can support an HT system.

With the widespread use of the WLAN and the diversification ofapplications using the WLAN, there is a recent demand for a new WLANsystem to support a higher throughput in comparison with a dataprocessing rate supported by the IEEE 802.11n. A very high throughput(VHT) WLAN system is a next version of the IEEE 802.11n WLAN system, andis one of IEEE 802.11 WLAN systems which have recently been proposed tosupport a data processing rate of above 1 Gbps in a medium accesscontrol (MAC) service access point (SAP).

The VHT WLAN system allows simultaneous channel access of a plurality ofVHT STAs for the effective use of a radio channel. For this, amulti-user multiple input multiple output (MU-MIMO)-based transmissionusing multiple antennas is supported. The VHT AP can perform spatialdivision multiple access (SDMA) transmission for transmittingspatial-multiplexed data to the plurality of VHT STAs. When data issimultaneously transmitted by distributing a plurality of spatialstreams to the plurality of STAs by using a plurality of antennas, anoverall throughput of the WLAN system can be increased.

Since a PPDU transmitted by the VHT AP and/or the VHT STA is transmittedthrough a plurality of spatial streams by using beamforming, in order toacquire data by using the PPDU, control information for the PPDU isrequired by the VHT STA and/or the VHT AP for receiving the PPDU. Thecontrol information may be transmitted by being included in thetransmitted PPDU. Although the control information is not significant interms of size and number, the control information is relativelyimportant since the control information is a basic element forinterpreting the PPDU for data acquisition. Accordingly, there is a needfor a method capable of transmitting the control information with highreliability.

SUMMARY OF THE INVENTION

The present invention provides a method for transmitting controlinformation with high reliability in a wireless local area network(WLAN) system and an apparatus supporting the method.

In an aspect, a method for transmitting control information in awireless communication system is provided. The method includestransmitting common control information including a multiple inputmultiple output (MIMO) indicator indicating single user-MIMO (SU-MIMO)or multi user-MIMO (MU-MIMO) to a receiver, generating first precodeddedicated control information by performing precoding on dedicatedcontrol information including information for the MU-MIMO by the use ofa first precoding matrix ,generating second precoded dedicated controlinformation by performing precoding on the first precoded dedicatedcontrol information by the use of a second precoding matrix andtransmitting the second precoded dedicated control information to thereceiver.

The first precoding matrix may be defined according to the number of allspatial streams for the MU-MIMO.

The second precoding matrix may be defined according to the number ofspatial streams allocated to the receiver.

The second precoding matrix may be selected from at least one columnvector of a discrete Fourier transform (DFT) matrix M_(DFT) expressed by

$M_{DFT} = \begin{bmatrix}1 & 1 & \ldots & 1 \\1 & e^{{- j}\frac{2\;\pi}{N}1} & \ldots & e^{{- j}\;\frac{2\;\pi}{N}{({N - 1})}} \\\vdots & \vdots & e^{{- j}\frac{2\;\pi}{N}{({n - 1})}{({m - 1})}} & \vdots \\1 & e^{{- j}\frac{2\;\pi}{N}{({N - 1})}} & \ldots & e^{{- j}\frac{2\;\pi}{N}{({N - 1})}{({N - 1})}}\end{bmatrix}$

where N is the number of spatial streams to be transmitted to thereceiver, n is a constant indicating a row, and m is a constantindicating a column.

The second precoding matrix may be determined alternately to the atleast one column vector of the DFT matrix in each of at least onesubcarrier used for transmission of the second precoded dedicatedcontrol information.

The second precoding matrix may be defined such that a phase shift isperformed on each of at least one subcarrier used for transmission ofthe second precoded dedicated control information.

The method may further include scrambling the dedicated controlinformation by using a scrambling code.

The scrambling code may be generated based on a unique identifier of thereceiver.

The common control information may include sub-information indicatingthe number of spatial streams allocated to the receiver and thescrambling code may be determined based on a value indicated by an indexallocated to the sub-information indicating the number of spatialstreams.

The scrambling code may be a pseudo noise (PN) sequence.

In another aspect, a wireless apparatus is provided. The wirelessapparatus includes a processor and a transceiver operationally coupledto the processor to transmit and receive a frame. The processor isconfigured for transmitting common control information including amultiple input multiple output (MIMO) indicator indicating singleuser-MIMO (SU-MIMO) or multi user-MIMO (MU-MIMO) to a receiver,generating first precoded dedicated control information by performingprecoding on dedicated control information including information for theMU-MIMO by the use of a first precoding matrix, generating secondprecoded dedicated control information by performing precoding on thefirst precoded dedicated control information by the use of a secondprecoding matrix and transmitting the second precoded dedicated controlinformation to the receiver.

According to the present invention, control information can betransmitted with high reliability by using a method for transmittingcontrol information related to multi-user multiple input multiple output(MU-MIMO) transmission by the use of spatial diversity.

In addition, a bit sequence constituting the control information relatedto MU-MIMO transmission is randomized through scrambling to decreaseinterference between stations (STAs).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a physical layer architecture of the institute ofelectrical and electronics engineers (IEEE) 802.11.

FIG. 2 is a diagram showing an example of a physical layer convergenceprocedure (PLCP) protocol data unit (PPDU) format used in a wirelesslocal area network (WLAN) system based on the IEEE 802.11n standard.

FIG. 3 is a diagram showing an example of a PPDU format that can be usedin a very high throughput (VHT) WLAN system.

FIG. 4 is a diagram showing a PPDU format in a VHT WLAN system.

FIG. 5 is a diagram showing a PPDU frame format according to anembodiment of the present invention.

FIG. 6 and FIG. 7 show examples of a PPDU format according to anembodiment of the present invention.

FIG. 8 is a flowchart showing a method of transmitting a PPDU accordingto an embodiment of the present invention.

FIG. 9 shows an example of bit scrambling applicable to an embodiment ofthe present invention.

FIG. 10 is a block diagram showing a wireless apparatus to which anembodiment of the present invention is applicable.

DETAILED DESCRIPTION OF THE INVENTION

A wireless local area network (WLAN) system according to an embodimentof the present invention includes at least one basic service set (BSS).The BSS is a set of stations (STAs) successfully synchronized tocommunicate with one another. The BSS can be classified into anindependent BSS (IBSS) and an infrastructure BSS.

The BSS includes at least one STA and an access point (AP). The AP is afunctional medium for providing a connection to STAs in the BSS throughrespective wireless media. The AP can also be referred to as otherterminologies such as a centralized controller, a base station (BS), ascheduler, etc.

The STA is any functional medium including a medium access control (MAC)and wireless-medium physical layer (PHY) interface satisfying theinstitute of electrical and electronics engineers (IEEE) 802.11standard. The STA may be an AP or a non-AP STA. Hereinafter, the STArefers to the non-AP STA unless specified otherwise.

The STA can be classified into a very high throughput (VHT)-STA, a highthroughput (HT)-STA, and a legacy (L)-STA. The HT-STA is an STAsupporting IEEE 802.11n. The L-STA is an STA supporting a previousversion of IEEE 802.11n, for example, IEEE 802.11a/b/g. The L-STA isalso referred to as a non-HT STA.

FIG. 1 shows an IEEE 802.11 physical layer (PHY) architecture.

The IEEE 802.11 PHY architecture includes a PHY layer management entity(PLME), a physical layer convergence procedure (PLCP) sub-layer 110, anda physical medium dependent (PMD) sub-layer 100. The PLME provides a PHYmanagement function in cooperation with a MAC layer management entity(MLME). The PLCP sub-layer 110 located between a MAC sub-layer 120 andthe PMD sub-layer 100 delivers to the PMD sub-layer 100 a MAC protocoldata unit (MPDU) received from the MAC sub-layer 120 under theinstruction of the MAC layer, or delivers to the MAC sub-layer 120 aframe received from the PMD sub-layer 100. The PMD sub-layer 100 is alower layer of the PDCP sub-layer and serves to enable transmission andreception of a PHY entity between two STAs through a radio medium. TheMPDU delivered by the MAC sub-layer 120 is referred to as a physicalservice data unit (PSDU) in the PLCP sub-layer 110. Although the MPDU issimilar to the PSDU, when an aggregated MPDU (A-MPDU) in which aplurality of MPDUs are aggregated is delivered, individual MPDUs andPSDUs may be different from each other.

The PLCP sub-layer 110 attaches an additional field includinginformation required by a PHY transceiver to the MPDU in a process ofreceiving the MPDU from the MAC sub-layer 120 and delivering a PSDU tothe PMD sub-layer 100. The additional field attached in this case may bea PLCP preamble, a PLCP header, tail bits required on a data field, etc.The PLCP preamble serves to allow a receiver to prepare asynchronization function and antenna diversity before the PSDU istransmitted. The PLCP header includes a field that contains informationon a PLCP protocol data unit (PDU) to be transmitted, which will bedescribed below in greater detail with reference to FIG. 2.

The PLCP sub-layer 110 generates a PLCP protocol data unit (PPDU) byattaching the aforementioned field to the PSDU and transmits thegenerated PPDU to a reception STA via the PMD sub-layer. The receptionSTA receives the PPDU, acquires information required for data recoveryfrom the PLCP preamble and the PLCP header, and recovers the data.

FIG. 2 is a diagram showing an example of a PPDU format used in a WLANsystem based on the IEEE 802.11n standard.

FIG. 2(a) shows a legacy PPDU (L-PPDU) format for a PPDU used in theexisting IEEE 802.11a/b/g.

An L-PPDU 210 includes an L-STF field 211, an L-LTF field 212, an L-SIGfield 213, and a data field 214.

The L-STF field 211 is used for frame timing acquisition, automatic gaincontrol (AGC) convergence, coarse frequency acquisition, etc.

The L-LTF field 212 is used for frequency offset and channel estimation.

The L-SIG field 213 includes control information for demodulation anddecoding of the data field 214.

The L-PPDU may be transmitted in the order of the L-STF field 211, theL-LTF field 212, the L-SIG field 213, and the data field 214.

FIG. 2(b) is a diagram showing an HT-mixed PPDU format in which an L-STAand an HT-STA can coexist. An HT-mixed PPDU 220 includes an L-STF field221, an L-LTF field 222, an L-SIG field 223, an HT-SIG field 224, anHT-STF field 225, a plurality of HT-LTF fields 226, and a data field227.

The L-STF field 221, the L-LTF field 222, and the L-SIG field 223 areidentical to those shown in FIG. 2(a). Therefore, the L-STA caninterpret the data field by using the L-STF field 221, the L-LTF field222, and the L-SIG field 223 even if the HT-mixed PPDU 220 is received.The L-LTF field 222 may further include information for channelestimation to be performed by the HT-STA in order to receive theHT-mixed PPDU 220 and to interpret the L-SIG field 223, the HT-SIG field224, and the HT-STF field 225.

The HT-STA can know that the HT-mixed PPDU 220 is a PPDU dedicated tothe HT-STA by using the HT-SIG field 224 located next to the L-SIG field223, and thus can demodulate and decode the data field 227.

The HT-STF field 225 may be used for frame timing synchronization, AGCconvergence, etc., for the HT-STA.

The HT-LTF field 226 may be used for channel estimation for demodulationof the data field 227. Since the IEEE 802.11n supports single user-MIMO(SU-MIMO), a plurality of the HT-LTF fields 226 may be configured forchannel estimation for each of data fields transmitted through aplurality of spatial streams.

The HT-LTF field 226 may consist of a data HT-LTF used for channelestimation for a spatial stream and an extension HT-LTF additionallyused for full channel sounding. Therefore, the number of the pluralityof HT-LTF fields 226 may be equal to or greater than the number ofspatial streams to be transmitted.

The L-STF field 221, the L-LTF field 222, and the L-SIG field 223 aretransmitted first so that the L-STA also can acquire data by receivingthe HT-mixed PPDU 220. Thereafter, the HT-SIG field 224 is transmittedfor demodulation and decoding of data transmitted for the HT-STA.

Up to fields located before the HT-SIG field 224, transmission isperformed without beamforming so that the L-STA and the HT-STA canacquire data by receiving a corresponding PPDU. In the subsequentlyfields, i.e., the HT-STF field 225, the HT-LTF 226, and the data field227, radio signal transmission is performed by using precoding. In thiscase, the HT-STF field 225 is transmitted so that an STA that receives aprecoded signal can consider a varying part caused by the precoding, andthereafter the plurality of HT-LTF fields 226 and the data field 227 aretransmitted.

Even if an HT-STA that uses 20 MHz in an HT WLAN system uses 52 datasubcarriers per OFDM symbol, an L-STA that also uses 20 MHz uses 48 datasubcarriers per OFDM symbol. Since the HT-SIG field 224 is decoded byusing the L-LTF field 222 in a format of the HT-mixed PPDU 220 tosupport backward compatibility, the HT-SIG field 224 consists of 482data subcarriers. The HT-STF field 225 and the HT-LTF 226 consist of 52data subcarriers per OFDM symbol. As a result, the HT-SIG field 224 issupported using ½ binary phase shift keying (BPSK), each HT-SIG field224 consists of 24 bits, and thus 48 bits are transmitted in total. Thatis, channel estimation for the L-SIG field 223 and the HT-SIG field 224is performed using the L-LTF field 222, and a bit sequence constitutingthe L-LTF field 222 can be expressed by Equation 1 below. The L-LTFfield 222 consists of 48 data subcarriers per one symbol, except for aDC subcarrier.

[Equation 1]L _(−26,26)={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1}

FIG. 2(c) is a diagram showing a format of an HT-Greenfield PPDU 230that can be used by only an HT-STA. The HT-GF PPDU 230 includes anHT-GF-STF field 231, an HT-LTF1 field 232, an HT-SIG field 233, aplurality of HT-LTF2 fields 234, and a data field 235.

The HT-GF-STF field 231 is used for frame timing acquisition and AGC.

The HT-LTF1 field 232 is used for channel estimation.

The HT-SIG field 233 is used for demodulation and decoding of the datafield 235.

The HT-LTF2 234 is used for channel estimation for demodulation of thedata field 235. Since the HT-STA uses SU-MIMO, channel estimation isrequired for each of data fields transmitted through a plurality ofspatial streams, and thus a plurality of HT-LTF2 fields 234 may beconfigured.

The plurality of HT-LTF2 fields 234 may consist of a plurality of dataHT-LTFs and a plurality of extension HT-LTFs, similarly to the HT-LTF226 of the HT-mixed PPDU 220.

Each of the data fields 214, 227, and 235 respectively shown in FIG.2(a), (b), and (c) may include a service field, a scrambled PSDU field,a tail bits field, and a padding bits field.

Unlike the IEEE 802.11n standard supporting the HT, the IEEE 802.11acrequires a higher throughput. It is called a very high throughput (VHT)to be distinguished from the HT, and 80 MHz bandwidth transmissionand/or higher bandwidth transmission (e.g., 160 MHz) are supported inthe IEEE 802.11ac. In addition, multi user-MIMO (MU-MIMO) is supported.

An amount of control information transmitted to STAs for MU-MIMOtransmission may be relatively greater than an amount of IEEE 802.11ncontrol information. For example, control information additionallyrequired for the VHT WLAN system may be information indicating thenumber of spatial streams that must be received by each STA, informationregarding modulation and coding of data transmitted for each STA, etc.Therefore, when MU-MIMO transmission is performed to provide datasimultaneously to a plurality of STAs, the control information to betransmitted may increase in amount according to the number of receptionSTAs.

In order to effectively transmit the increased amount of controlinformation to be transmitted, among a plurality of pieces of controlinformation required for MU-MIMO transmission, control informationrequired commonly for all STAs and control information requiredindividually for the STAs may be transmitted by distinguishing the twotypes of control information. Hereinafter, this will be described ingreater detail by reference to a PPDU format in a WLAN system supportingthe IEEE 802.11ac. An STA implies a VHT-STA in the followingdescription.

FIG. 3 is a diagram showing an example of a PPDU format that can be usedin a VHT WLAN system.

Referring to FIG. 3, a PPDU 300 includes an L-STF field 310, an L-LTFfield 320, an L-SIG field 330, a VHT-SIGA field 340, a VHT-STF field350, a VHT-LTF field 360, a VHT-SIGB field 370, and a data field 380.

A PLCP sub-layer converts a PSDU delivered from a MAC layer into a datafield by attaching required information to the PSDU, generates the PPDU300 by attaching various fields such as the L-STF field 310, the L-LTFfield 320, the L-SIG field 330, the VHT-SIGA field 340, the VHT-STFfield 350, the VHT-LTF field 360, the VHT-SIGB field 370, etc., andtransmits the PPDU 300 to one or more STAs through a PMD layer.

The L-STF field 310 is used for frame timing acquisition, AGCconvergence, coarse frequency acquisition, etc.

The L-LTF field 320 is used for channel estimation for demodulation ofthe L-SIG field 330 and the VHT-SIGA field 340.

The L-SIG field 330 is used when the L-STA receives the PPDU to acquiredata.

The VHT-SIGA field 340 is common control information required forVHT-STAs which are MU-MIMO paired with an AP, and includes controlinformation required to interpret the received PPDU 300. The VHT-SIGAfield 340 includes information for a spatial stream for each STA,bandwidth information, identification information for indicating whetherspace time block coding (STBC) is used, a group identifier (i.e.,identification information for an STA group), information on an STA towhich each group identifier is allocated, and information related to ashort guard interval (GI). Herein, the group identifier may includewhether a currently used MIMO transmission scheme is MU-MIMO or SU-MIMO.

The VHT-STF field 350 is used to improve performance of AGC estimationin MIMO transmission.

The VHT-LTF field 360 is used when an STA estimates a MIMO channel.Since a VHT WLAN system supports MU-MIMO, the VHT-LTF field 360 can beconfigured by the number of spatial streams through which the PPDU 300is transmitted. In addition, when full channel coding is supported andis performed, the number of VHT LTFs may increase.

The VHT-SIGB field 370 includes dedicated control information requiredwhen the MU-MIMO paired STA receives the PPDU 300 to acquire data.Therefore, the STA may be designed such that the VHT-SIGB field 370 isdecoded only when the common control information included in theVHT-SIGB field 370 indicates that a currently received PPDU istransmitted using MU-MIMO transmission. On the contrary, the STA may bedesigned such that the VHT-SIGB field 370 is not decoded when the commoncontrol information indicates that the currently received PPDU is for asingle STA (including SU-MIMO).

The VHT-SIGB field 370 includes information on each STA's modulation,encoding, and rate-matching. The VHT-SIGB field 370 may have a differentsize according to a MIMO transmission type (i.e., MU-MIMO or SU-MIMO)and a channel bandwidth used for PPDU transmission.

The VHT WLAN system employs the VHT-SIGA field 340 including commoncontrol information commonly applied to a plurality of STAs and theVHT-SIGB field 370 including dedicated control information individuallyapplied to the respective STAS as described above for the effectivesupport of MU-MIMO. Since the VHT-SIGA field 340 is allocated 48 datasubcarriers per OFDM symbol similarly to the L-STF field 310, the L-LTFfield 320, and the L-SIG field 330 for backward compatibility, the L-LTFfield 320 is used for channel estimation. However, the VHT-STF field 350and the VHT-LTF field 360 are transmitted after transmission of theVHT-SIGA field 340, and for this, 52 data subcarriers are used per OFDMsymbol. Likewise, since the VHT-SIGB field 370 is transmitted using 52data subcarriers, channel estimation of the VHT-SIGB field 370 isperformed by using the VHT-LTF field 360. If it is assumed that theVHT-LTF field 360 and the HT-LTF field 226 of FIG. 2(b) use the same bitsequence, it can be expressed by Equation 2 below, and the bit sequenceconsists of 52 data subcarriers per one symbol except for a DCsubcarrier.

[Equation 2]VHTLTF_(−28,28)={1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,1,−1,−1,1,1,−1,1,−1,1, −1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1,−1,−1}

Since Equation 1 and Equation 2 above are different from each other, ifthe VHT-SIGA field 340 is transmitted using ½ (rotated) BPSK, the fieldhas a size of 48 bits, and if the VHT-SIGB field 370 is transmittedusing ½ (rotated) BPSK, the VHT-SIGB field 370 has a size of 26 bits.

The L-LTF field 320, consisting of 48 data subcarriers (i.e.,subcarriers indexed with −26 to −1 and 1 to 26, where 4 subcarrierscorrespond to pilots) per symbol, may be used for channel estimation ofVHT-SIGA field 340, and the VHT-LTF field 360 consisting of 52 datasubcarriers (i.e., subcarriers indexed with −28 to −1 and 1 to 28, where4 subcarriers correspond to pilots) per symbol may be used for channelestimation of VHT-SIGB field 370. A diagram of FIG. 4 for showing a PPDUformat transmitted or received in a VHT WLAN system may be used hereinby reference.

Referring to FIG. 4, channel estimation is performed based on an L-LTFfield 410 when an STA receives an L-SIG field 420 and a VHT-SIGA field430 which are indicated by a dotted shaded area. The VHT-SIGA field 430is allocated to two symbols, and has a size of 48 bits.

On the other hand, when the STA receives a VHT-SIGB field 450 indicatedby a slash shadow area, channel estimation is performed based on aVHT-LTF field 440 (i.e., VHT LTF1, VHT-LTF2, . . . , VHT-LTFx). TheVHT-SIGB field 450 is allocated to one symbol, and has a size of 26bits.

When transmitting a data field 460, a modulation and coding scheme (MCS)may be optionally included in the VHT-SIGA field 430 and/or the VHT-SIGBfield 450. In addition, the L-LTF for the VHT-SIGA field is transmittedomni-directionally, and the VHT-LTF 440 for the VHT-SIGB field 450 istransmitted by performing beamforming based on a precoding matrix.

52 data subcarriers per symbol is an exemplary case where a parameterused in an HT WLAN system is directly used in a VHT WLAN system. If theVHT WLAN system is newly designed, the number of data subcarriers persymbol may be greater than 52, and a new VHT-LTF may be defined. Thatis, if more than 48 data subcarriers can be transmitted per symbol, theVHT-SIGB field greater than 24 bits may be transmitted and thus theVHT-SIGB field and the VHT-SIGA field may use different LTF bitsequences for channel estimation.

Referring back to FIG. 3, the VHT-SIGA field 340 uses a cyclic shiftingdelay (CSD) in a transmission (Tx) antenna domain (or time domain) sothat it can be received by all STAs that are paired in MU-MIMOtransmission. On the other hand, since the VHT-SIGB field 370 includesdedicated control information that must be received by a specific STAfor receiving the data field 380, the VHT-SIGB field 370 may betransmitted by performing beamforming based on a precoding matrix unlikethe VHT-SIGA field 340.

When the data field 380 is transmitted through a plurality of spatialstreams, the VHT-SIGB field 370 is also transmitted by performingbeamforming by the use of the same precoding matrix as that used in thedata field 380. Unlike data containing information that can betransmitted through the plurality of spatial streams, a fixed amount ofcontrol information may be included in the VHT-SIGB field 370.Therefore, the VHT-SIGB field 370 may be preferably transmitted throughone spatial stream instead of being transmitted through a spatial streamby performing beamforming based on the precoding matrix.

If the VHT-SIGB field 370 is transmitted through one spatial streamamong the plurality of spatial streams through which the data field 380is transmitted, a specific spatial stream through which the VHT-SIGBfield 370 is transmitted must be pre-agreed between a transmitting endand a receiving end. This can be implemented by assigning anidentifiable index to all MU-MIMO spatial streams transmitted to aplurality of STAs and by allowing the VHT-SIGB field 370 to betransmitted through a spatial stream having a first index value of aspatial stream used for each STA or having a specific index value.

If at least one or more spatial streams through which the data field 380is currently being transmitted in a pre-coded virtual spatial domain arecalled an available sub-space, the VHT-SIGB field 370 may be transmittedby selecting a specific spatial stream which is a specific sub-space ofthe available sub-space. However, in this case, the sub-space may not anoptimal sub-space for transmission. In addition, when transmission isperformed by selecting only one specific sub-space from the availableplurality of sub-spaces, if transmissible maximum power exists in thatsub-space, maximum possible performance may not be acquired. This isbecause the maximum transmissible transmission power is not fullyutilized in the transmitting end.

On the other hand, in a case where there is no restriction on themaximum transmissible power in each sub-space, if the VHT-SIGB field 370is transmitted through only one sub-space among the plurality ofsub-spaces through which data is being transmitted, power transmittedthrough that sub-space may be further used for transmission by thenumber of sub-spaces not used in the VHT-SIGB field 370. For example, ifthe data field 380 is transmitted through two spatial streams and theVHT-SIGB field 370 is transmitted through a first spatial stream betweenthe two spatial streams, power of a signal for transmitting the VHT-SIGBfield 370 may be increased two times. In this case, a spatial stream notused in transmission of the VHT-SIGB field 370 may be transmitting byinserting NULL. This implies that there is no signal transmitted byusing a second spatial stream in the above example.

In a case where the VHT-SIGB field 370 is transmitted through a onespecific spatial stream among a plurality of spatial streams used fordata field transmission, transmission efficiency of the VHT-SIGB field370 may not be optimized even if signal power increases. This is becausethe transmission efficiency of the VHT-SIGB field 370 may be determinedby performance of the spatial stream itself. To solve this problem, amethod of transmitting the VHT-SIGB field 370 by using all spatialstreams through which the data field 380 is transmitted is proposed.This may be implemented by using a method of transmitting the VHT-SIGBfield 370 by additionally applying a different precoding vector for theVHT-SIGB field 370 and the data field 380 transmitted to a virtualspatial stream domain. Hereinafter, an embodiment of the presentinvention will be described in greater detail with reference to theaccompanying drawings.

FIG. 5 is a diagram showing a PPDU frame format according to anembodiment of the present invention.

Referring to FIG. 5, a PPDU 500 includes an L-STF field 510, an L-LTFfield 520, an L-SIG field 530, a VHT-SIGA field 540, a VHT-STF field550, a VHT-LTF field 560, a VHT-SIGB field 570, and a data field 580.The fields included in the PPDU 500 have the same meaning and usage asthose explained above, and thus details descriptions thereof will beomitted.

When N_(SS) denotes the number of spatial streams used for transmissionof the data field to a specific STA, the VHT-SIGB field 570 may bemapped to a plurality of spatial streams by applying a precoding vectorhaving a size of N_(SS)xl to control information corresponding to onespatial stream.

A spatial stream through which the VHT-SIGB field 570 and the data field580 are transmitted corresponds to a sub-space domain virtualizedprimarily by a precoding matrix Qk. Therefore, when a precoding vectorV_(k) is secondarily applied for transmission of the VHT-SIGB field 570,it corresponds to re-virtualization of a domain which has already beenvirtualized by the precoding matrix Q_(k). Since a sub-space throughwhich the VHT-SIGB field 570 is transmissible is identical to asub-space through which the data field 580 is transmitted, when theprecoding vector V_(k) is applied, it implies that a signal istransmitted by using only some of all sub-spaces. Therefore, there is aneed for a method of acquiring spatial diversity while applying theprecoding vector V_(k).

In the present invention, in order to perform transmission with thespatial diversity by applying the precoding vector V_(k), the randomvector V_(k) pre-agreed between a transmitting end and a receiving endcan be applied for each frequency subcarrier. A method of alternatelyusing a column vector of a discrete Fourier transform (DFT) matrix isproposed so that the vector V_(k) that changes in a frequency axis istransmitted across all sub-spaces to be spanned. This will be describedby reference to Equation 3 and Equation 4 below.

$\begin{matrix}{M_{DFT} = \begin{bmatrix}1 & 1 & \ldots & 1 \\1 & e^{{- j}\frac{2\;\pi}{N}1} & \ldots & e^{{- j}\frac{2\;\pi}{N}{({N - 1})}} \\\vdots & \vdots & e^{{- j}\frac{2\;\pi}{N}{({n - 1})}{({m - 1})}} & \vdots \\1 & e^{{- j}\frac{2\;\pi}{N}{({N - 1})}} & \ldots & e^{{- j}\frac{2\;\pi}{N}{({N - 1})}{({N - 1})}}\end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack \\{\mspace{79mu}{v_{k} = \left\{ {\begin{bmatrix}1 \\1 \\1\end{bmatrix},\begin{bmatrix}1 \\e^{{- j}\frac{2\;\pi}{3}} \\e^{{- j}\frac{4\;\pi}{3}}\end{bmatrix},\begin{bmatrix}1 \\e^{{- j}\frac{4\;\pi}{3}} \\e^{{- j}\;\frac{8\;\pi}{3}}\end{bmatrix}} \right\}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Equation 3 above expresses a normal DFT matrix, and Equation 4 aboveexpresses the precoding vector V_(k) when the number of spatial streamsreceived by a specific STA is 3. The precoding vector V_(k) expressed inEquation 4 above is repeated every three frequency subcarriers.Therefore, if the VHT-SIGB field 570 spans three subcarriers,transmission is performed by alternating all sub-spaces through whichthe data field 580 is transmitted, and in this manner, spatial diversitycan be acquired.

Meanwhile, when a column vector included in a specific unitary matrix isalternately used as the precoding vector V_(k) to be applied to theVHT-SIGB field 570 as described above, if the total number of frequencysubcarriers is indivisible by the number of column vectors of thespecific unitary matrix, some of the all sub-spaces through which theVHT-SIGB field 570 is transmitted may be transmitted more than othersub-spaces. Therefore, as a method of evenly transmitting the allsub-spaces through which the VHT-SIGB field 570 is transmissible to themaximum extent possible, additional cyclic delay diversity may beapplied.

In order to apply the additional cyclic delay diversity in thetransmission of the VHT-SIGB field 570, a phase shift per frequencysubcarrier may be performed for each vector element. For this, toperform the phase shift per frequency subcarrier, multiplication may beperformed while increasing an absolute value of power of an exponentialfunction with a base of a natural constant. For example, if N_(F),denotes the total number of frequency subcarriers, when spatial streamsthat must be received by a specific STA are indexed from 0, a valueacquired by performing a phase shift to be applied to a spatial streamhaving a (k-1)th index value can be expressed by

$e^{{- j}\;\frac{2\;\pi}{N_{F}}{({k - 1})}n}.$Herein, n is a cyclic delay value, and more specifically, may be 1 or 2.It can be expressed in a normal vector form as shown in Equation 5below.

$\begin{matrix}{v_{cd} = \begin{bmatrix}1 \\e^{{- j}\frac{2\;\pi}{N_{F}}{k \cdot n}} \\e^{{- j}\frac{2\;\pi}{N_{F}}{k\; \cdot 2}\; n} \\\vdots \\e^{{- j}\frac{2\;\pi}{N_{F}}{k \cdot {({N_{SS} - 1})}}n}\end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Herein, N_(SS) denotes the total number of spatial streams required fortransmission of the VHT-SIGB 570 and the data field 580, k denotes anindex value of a frequency subcarrier, N_(F) denotes the total number ofall frequency subcarriers, and n denotes a cyclic delay value.

A PPDU format based on the transmission method according to theaforementioned embodiment of the present invention can be expressed byFIG. 6 and FIG. 7.

FIG. 6 and FIG. 7 show examples of the PPDU format according to theembodiment of the present invention. Regarding the PPDU format, a VHTLTF field is expressed in detail in FIG. 6, and a VHT-SIGB field isexpressed in detail in FIG. 7.

Referring to FIG. 6, the VHT LTF is transmitted by multiplying anorthogonal transform matrix to perform channel estimation for eachspatial stream. In the orthogonal transform matrix, the number ofcolumns may be determined by the number of OFDM symbols used fortransmission of the VHT-LTF, and the number of rows may be determined bythe number of spatial streams allocated to a specific STA which isMU-MIMO paired. In FIG. 6, a VHT-LTF to be transmitted to an STA 1 istransmitted by using 4 OFDM symbols through 3 spatial streams, and thusthe orthogonal transform matrix may be a 3×4 matrix.

Referring to FIG. 7, it is enough for the VHT-SIGB field to be normallytransmitted to a specific STA. The VHT-SIGB field may be transmitted bymultiplying a specific transform vector since spatial multiplexing to aplurality of spatial streams is not necessary. The specific transformvector may be the aforementioned additional precoding vector V_(k), ormay be a precoding vector to which a cyclic delay vector is additionallyapplied. The number of rows of the transform vector may be determined bythe number of spatial streams allocated to the specific STA which isMU-MIMO paired. In FIG. 7, the VHT-SIGB field to be transmitted to theSTA 1 does not have to be subjected to spatial multiplexing using 3spatial streams, and thus may be a 3×1 vector matrix.

FIG. 8 is a flowchart showing a method of transmitting a PPDU accordingto an embodiment of the present invention.

Referring to FIG. 8, an access point (AP) 810 transmits common controlinformation to an STA 820 (S810). The common control information may betransmitted by being included in a VHT-SIGA field of the PPDU.

The AP 810 performs precoding on dedicated control information to betransmitted to the STA 820 by using a first precoding matrix to generatefirst precoded dedicated control information (S820). A first precodingmatrix is a matrix for precoding data to be transmitted to the STA 820by using a MIMO scheme. The generation of the first precoded dedicatedcontrol information can be implemented by performing precoding on aVHT-SIGA field including the dedicated control information by the use ofthe first precoding matrix.

The AP 810 performs precoding on the first precoded dedicated controlinformation by using a second precoding matrix to generate secondprecoded dedicated control information (S830). The generation of thesecond precoded dedicated control information can be implemented byperforming precoding on a first precoded VHT-SIGA field by using asecond precoding matrix.

The AP 810 transmits the second precoded dedicated control informationto the STA 820 (S840).

The AP 810 generates precoded data by using the first precoding matrix(S850).

The AP 810 transmits the precoded data to the STA 820 (S860).

If common control information, more specifically, a group identifier(ID) included in the common control information, indicates MU-MIMOtransmission, the STA decodes the second precoded dedicated controlinformation and thereafter interprets precoded data. If the group IDincluded in the common control information indicates single user (SU)transmission, the second precoded dedicated control information may notbe decoded.

Referring back to FIG. 5, since the VHT WLAN system supports MU-MIMOtransmission, the VHT-SIGB fields 570 to be transmitted to a pluralityof different STAs are transmitted respectively to a plurality ofdifferent STAs. In this case, the VHT-SIGB field 570 transmitted to eachSTA paired to the AP may be generated in similar cases. The VHT-SIGBfield 570 includes a tail bit, a frame length, and an MCS value fordifferent STAs. When the AP provides a service to many STAs that wait toreceive the service, there may be a case where a possibility ofproviding the same-length frame is high, the tail bit is similar sinceit is always 0, and several bits of the MCS value are different. In thiscase, similar bits are encoded, and thus spatial interference may occurin which the VHT-SIGB field 570 transmitted for a specific STA has aneffect on another VHT-SIGB field transmitted for another STA. As aresult, an unnecessary field may be detected rather than detecting anecessary VHT-SIGB field.

To solve such as problem, the VHT-SIGB field transmitted to each ofpaired STAs may be subjected to bit scrambling. The bit scrambling maybe performed before or after encoding the VHT-SIGB field.

Hereinafter, a scrambling method applicable to the present inventionwill be described. A scrambling code used for scrambling may begenerated in various manners, and applicable examples will be describedhereinafter.

First, the scrambling code may be generated based on an association ID(AID) which is a unique ID of each STA. The AP allocates the AID to eachSTA within a BSS. Each STA is identifiable by the AID since there is nopossibility that the AID overlaps in the BSS. Therefore, when thescrambling code is generated by using the AID as a scrambling initiator,a different scrambling code may be allocated to each STA.

Second, the scrambling code may be generated based on a group index forMU-MIMO transmission. The present embodiment proposes to use an N_(STS)field included in the VHT-SIGA field as one index. It is assumed that aset of scrambling codes is pre-defined. This will be described ingreater detail with reference to the accompanying drawing.

FIG. 9 shows an example of bit scrambling applicable to an embodiment ofthe present invention.

Referring to FIG. 9, a PPDU 900 is transmitted to an STA 1 and an STA 2by using a MU-MIMO scheme. The PPDU 900 includes a VHT-SIGA field 910for the STA 1 and the STA 2. The VHT-SIGA field 910 includes an N_(STS)subfield 911 for the STA 1 and an N_(STS) subfield 912 for the STA 2.The N_(STS) subfields 911 and 912 are fields for indicating the numberor position of space-time streams allocated to each STA. The number ofN_(STS) subfields 911 and 912 to be included may be equal to the numberof STAs which are MU-MIMO paired to an AP. In FIG. 9, the number of TheN_(STS) subfields 911 and 912 that can be included in the VHT-SIGA field910 may be two for the STA 1 and the STA 2. Therefore, the N_(STS)subfields 911 and 912 may be used as indicators for identifying theMU-MIMO paired STAs.

Index values are assigned to the N_(STS) subfields 911 and 912 in theVHT-SIGA field 910. Each index value may match to a unique scramblingcode. The scrambling code may correspond to information known to the APand all STAs which are MU-MIMO paired to the AP.

In the generation of the PPDU 900, when the VHT-SIGB field 920 isscrambled, the AP may select a scrambling code based on positions of theN_(STS) subfields 911 and 912 included in the VHT-SIGA field 910. In thefigure, the N_(STS) subfield 911 for the STA 1 is located at a firstposition in the VHT-SIGA field 910 and a value ‘1’ is assigned as anindex value. Therefore, ‘1 1 1 1’ is selected as a scrambling code to beapplied to the VHT-SIGB field 921 to be transmitted to the STA 1.Likewise, the N_(STS) subfield 912 for the STA 2 is located in a secondposition in the VHT-SIGA field 910, and a value ‘2’ is assigned as anindex value. Therefore, ‘1 0 1 0’ may be selected as a scrambling codeto be applied to the VHT-SIGB field 922 to be transmitted to the STA 2.The AP scrambles each of the VHT-SIGB fields 911 and 912 by using acorresponding scrambling code.

The MU-MIMO paired STA receives the PPDU 900, and confirms the positionsof the N_(STS) subfields 911 and 912 for the STA within the VHT-SIGAfield 910. The STA confirms an index value by using the positions of theN_(STS) subfields 911 and 912 and thus can know scrambling codes appliedto the VHT-SIGB fields 921 and 922. Therefore, the STA can descramblethe VHT-SIGB fields 921 and 922 for the STA.

Although there are four types of scrambling codes each of which has asize of 4 bits in the example of FIG. 9, the number of types ofscrambling codes may be less than or greater than 4.

Third, a pseudo noise (PN) sequence may be used as a scrambling code.The AP transmits a PPDU by applying the PN sequence to a VHT-SIGB field.In addition, the PN sequence used herein may be reported to each STA.Although ‘10110111000’ is used as the PN sequence in the WLAN standard,this is for exemplary purposes only, and thus another sequence may bedefined and used. In addition, if there is a plurality of PN sequences,each of different PN sequences may be applied as a scrambling code.

FIG. 10 is a block diagram showing a wireless apparatus to which anembodiment of the present invention is applicable. A wireless apparatus1000 may be an AP or an STA.

Referring to FIG. 10, the wireless apparatus 1000 includes a processor1010, a memory 1020, and a transceiver 1030. The transceiver 1030transmits and/or receives a radio signal, and implements an IEEE 802.11physical layer. The processor 1010 is operationally coupled to thetransceiver 1030, and implements a physical layer for implementing theembodiment of the present invention shown in FIG. 3 to FIG. 9 in orderto transmit a PPDU.

The processor 1010 and/or the transceiver 1030 may include anapplication-specific integrated circuit (ASIC), a separate chipset, alogic circuit, and/or a data processing unit. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememory 1020 and may be performed by the processor 1010. The memory 1020may be located inside or outside the processor 1010, and may be coupledto the processor 1010 by using various well-known means.

What is claimed is:
 1. A method for transmitting data in a wirelesslocal area network, the method comprising: receiving, by a receivingstation, a physical layer protocol data unit (PPDU) from a transmittingstation, the PPDU including a first signal field, the first signal fieldincluding a bandwidth in which the PPDU is received; decoding the PPDU,wherein, when the PPDU is used for single-user (SU) transmission, thePPDU is decoded by assuming that the PPDU does not include a secondsignal field, and, when the PPDU is used for multi-user (MU)transmission, the PPDU is decoded by assuming that the PPDU includes thesecond signal field, wherein the second signal field includes amodulation and coding scheme (MCS) used for the receiving station, andwherein a size of the second signal field is determined based on thebandwidth indicated by the first signal field.
 2. The method of claim 1,wherein the PPDU further includes a long training symbol (LTF) formultiple-input multiple-output (MIMO) channel estimation, and whereinthe first signal field includes information on a guard interval of theLTF.
 3. The method of claim 1, wherein the first signal field furtherincludes information on at least one spatial stream allocated to thereceiving station.
 4. A device for a wireless local area network, thedevice comprising: a transceiver configured to transmit and receiveradio signals; and a processor operatively coupled to the transceiverand configured to: instruct the transceiver to receive a physical layerprotocol data unit (PPDU) from a transmitting station, the PPDUincluding a first signal field, the first signal field including abandwidth in which the PPDU is received, decode the PPDU, wherein, whenthe PPDU is used for single-user (SU) transmission, the PPDU is decodedby assuming that the PPDU does not include a second signal field, and,when the PPDU is used for multi-user (MU) transmission, the PPDU isdecoded by assuming that the PPDU includes the second signal field,wherein the second signal field includes a modulation and coding scheme(MCS) used for the device, and wherein a size of the second signal fieldis determined based on the bandwidth indicated by the first signalfield.
 5. The device of claim 4, wherein the PPDU further includes along training symbol (LTF) for multiple-input multiple-output (MIMO)channel estimation, and wherein the first signal field includesinformation on a guard interval of the LTF.
 6. The device of claim 4,wherein the first signal field further includes information on at leastone spatial stream allocated to the device.